The expanding solar wind also exhibits a turbulent character that in some respects resembles the well known hydrodynamic turbulence described by [1]}. In particular, a key element in the Alfvénic turbulence is the presence of counter-propagating Alfvén waves, which non-linearly interact to form the turbulent cascade [2]}. Alfvén waves generated due to photospheric motions propagate away from the Sun, but there are several suggested mechanisms on how to generate sunward travelling waves. First, the inhomogeneity of the solar corona causes Alfvén waves to partially reflect, which enables turbulent heating [3]}, [4]}, [5]}, [6]}, [7]}, [8]}. Recent studies [9]} have shown that phase mixing caused by the Alfvén velocity being inhomogenous perpendicular to the magnetic field lines [10]}, [11]}, [12]} can also generate turbulent heating. Finally, large amplitude Alfvén waves are subject to the parametric decay instability (PDI) in compressible and low beta plasma, resulting in Alfvén-wave turbulence [13]}, [14]}, [15]}, [16]}, [17]}, [18]}.
